1. Field of the Invention
The invention disclosed herein relates to correcting systematic errors in a sensor. In particular, the present disclosure is an apparatus and method for correcting systematic errors in a sensor for measuring gravity.
2. Description of the Related Art
Exploration and production of hydrocarbons generally requires precise and accurate measurements of earth formations, which may contain reservoirs of the hydrocarbons. Many types of parameters related to earth formations are typically measured. One important type of parameter is gravitational acceleration.
By knowing the amount of gravitational acceleration present at a position in a borehole penetrating an earth formation, it is possible to determine the true vertical depth of the position. In addition, contents of a reservoir can be monitored by measuring a change in gravitational acceleration at a position in the borehole.
Sensors are generally used to measure a physical parameter such as gravitational acceleration or the force of gravity. Parameters such as gravity may be difficult to measure due to parameter values being very low or environmental constraints being imposed upon the sensor. In an effort to sense difficult to measure parameters, some sensors have a complex structure. As a result, these sensors can be prone to inaccuracies known as systematic errors.
Systematic errors are biases in a measurement or in an output of a sensor. The biases are such that the mean of many measurements differs significantly from the actual value of the parameter being measured. Systematic errors arise when the amount of error and the associated sign are unknown.
Systematic errors are very difficult to account for because the effect of these errors are only observable if these errors can be removed. Repeated measurements or averaging large numbers of measurements cannot be used to remove systematic errors.
There are several types of systematic errors. One type is “offset bias.” With offset bias, the output of a sensor is offset a fixed amount from the true value throughout a range of values. Thus, when a true value is zero, the output of the sensor measuring the value indicates the fixed amount of offset, such as five units for example. When a true value such as 100 units is measured, the output of the sensor will be 105 units.
Another type of systematic error is proportional error, which may also be referred to as scaling factor or scaling sensitivity error. Proportional error is related to a proportion of the true value being measured. For example, if the true value of a force being measured is 100 units and the proportional error is plus ten percent, then the output of the sensor will be 110 units. If the true force being measured is ten units, then the output of the sensor will be 11 units.
In order to be able to perform accurate and precise measurements, it is important to be able to remove systematic errors.
Therefore, what are needed are techniques to remove or correct for systematic errors in sensors. In particular, the techniques need to be applied to sensors measuring the force of gravity in a borehole penetrating the earth.